1. We have a guy whose job is to clean windows.
2. We have a guy whose job it is to clean windows.

Comment 1:

[L]et's say you have a secretary at a company who, when clients come in for a meeting, is the person who is supposed to bring in coffee.

You generally wouldn't say that “we have a woman whose job is to bring coffee to client meeting,” because it makes it sound like she's a full-time coffee-bringer.

However, you could say that “we have a woman whose job it is to bring coffee to client meetings,” because when that task has to be done, she's the one whose responsibility it is.

Comment 2:

1. (?) His job is to clean windows.
2. It is his job to clean windows.
3. ? To clean windows is his job.

In the first sentence his job is subject, because it comes before the verb. ... It simply describes his job entails, what he's probably doing everyday. ...

( ... ... )

Sentence 2 looks most idiomatic. I think the phrase “it is his job” is a fixed expression with a slightly different connotation: it is his duty to clean windows. You could say it puts focus on it is his job, presents it as the new and salient information─this as opposed to sentence 1, in which focus is on to clean windows.

The reason why this indicates different focus is that, in 2, to clean windows is subject, which is expressed by the place-holder subject it that points forward to to clean windows. In a regular sentence, the subject is topic (non-focus, known information) and the predicate has focus, so that fits nicely. In sentence 1, the predicate was is to clean windows, and indeed that is what the focus was on.

In sentence 3, to clean windows is subject; it is of the same type as sentence 2, except that the deferred subject is filled in at the place of the place-holder subject it. But then we are troubled by an entirely different problem: English usually doesn't like it when an infinitive with to is used as the (direct) subject. In this case,, it doesn't look too good; that is exactly why place-holder subjects are use at all. We could use the gerund instead to get an acceptable sentence:

3.a. Cleaning windows is his job.

This would be just as fine as sentence 2, with similar focus on is his job.

Now let's return to the relative clauses:

We have a guy whose job is to clean windows.

This would be of type 1 above, if you consider word order. It may be acceptable, but it doesn't sound as good as type 2 in formal language, at least not to me. This is probably for the same reason as concerning sentence 1 above.

We have a guy whose job it is to clean windows.

This of type 2, and it sounds better. That is probably because it is his job is a fixed expression and focus is in the right place. It may not be entirely evident that what applies to a main clause carries over to a relative clause, but I think it is fairly reasonable hypothesis.

자료 2: 다른 예문들

I know a guy whose job is to teach English.

I know a guy whose job it is to teach English.

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